Such optically variable surface patterns with microscopic relief structures can be used, for example, to protect against forgery and to mark objects of all kind in a prominent manner, being useful in particular with securities, identification papers, means of payment and similar objects to be protected.
The embossed relief structures with an optically active covering cause diffraction of incident light falling thereon. The diffraction characteristics of these relief structures are determined, among other things, by the spacial frequency, i.e. by the number of lines per millimeter, by the profile configuration of the relief structure and by the amplitude or differences in height in the relief structure as well as by the azimuthal orientation of the relief structure on the object to be protected.
The relief structures can be provided with cross-sectional configurations of known periodic functions for example, with spacial frequencies of over 10 lines per mm that are effective for the diffraction of visible light. Due to manufacturing limitations, the practical usable range is limited to approximately 5000 lines/mm. However, cross-sectional configurations with aperiodic functions containing locally a mixture of spacial frequencies in that range, such as matte structures for instance, can also be used. The height difference selected for such relief structures lies typically between 50 nm and 10,000 nm.
The diffraction properties of the different relief structures are described in R. Petit, Electromagnetic Theory of Gratings, Springer Verlag, Berlin Heidelberg New York 1980, for example.
These structures can be produced economically by deforming a layer of synthetic thermoplastic material by means of a heatable embossing die which carries a negative of the desired relief structure as is described in U.S. Pat. No. 4,184,700.
An optically variable surface pattern as well as another method for the production of a master structure for the embossing die to emboss such surface patterns are described in the Swiss Patent application 00805/88-4 which corresponds to U.S. Pat. No. 311,596 filed on Feb. 15, 1989.
On the other hand holograms are known for the production of three-dimensional impressions. The optical information concerning every point of a holographically registered, diffusely reflecting object is distributed over the entire active surface so that a reduction of the hologram surface reduces the visible image portion only to an insignificant degree but reduces the usable range of the angle of view.
A process to obtain a relief structure from a hologram is described in U.S. Pat. No. 4,094,575. This relief structure can be embossed into a film of synthetic material and can be reproduced many times. Such structures are difficult to identify under illumination by means of area lights such as fluorescent lights, for example.
It is an object of the present invention to provide a planar light-modifying pattern changing in a predetermined manner with at least two different graphically formed images visible under different angles of view which is difficult to copy and the authenticity of which can be recognized under normal light, whether daylight or artificial light, even by persons not specially trained.